Fablehaven is the name of the book, and Fablehaven is the name of the magical preserve where most of the action happens. Coincidence? We think not.
Let's start by breaking down the word: the fable part tells us that there'll be creatures from long-ago stories involved. And the haven part tells us that the place is a refuge, a safe spot for critters that the world is no longer able to handle. Put the two together, and you get the interesting notion that the beings originating in myth and magic need a new place to call home.
Why not name the book Kendra and Seth's Excellent Adventures? Or Kendra in Wonderland? Because the place itself is pretty important. We spend a lot of time tagging along with Kendra and Seth as they decode the mysteries leading them to figure out that, yes, this place is filled with magical creatures—plus the place is important to their futures.
Kendra and Seth stand to inherit Fablehaven from their grandparents, which would be a totally life-changing thing for these kids. Kendra uses this fact to help convince Grandma to let her and Seth come on the dangerous expedition to rescue Grandpa from Muriel, saying:
"If we're supposed to possibly inherit this place someday, you won't always be able to protect us from danger […] Wouldn't it be a good experience for us to watch you and Hugo handle the situation?" (15.154)
It's kind of ironic that Fablehaven exists to keep magical animals safe from the outside world… yet it's not always safe for the humans who are there to keep the magical animals safe. The title of the book helps reinforce that dual sense of safety versus danger, keeping us wondering what'll happen to Kendra and Seth as they're plopped down right in the middle of it.